There is a need for surgical repair devices such as bone plates, screws, staples, and other tissue and fracture fixation devices which are bioabsorbable. Suitable polymeric materials are desired which are readily and safely absorbed by the body, a process also known as bioresorption, at a rate which enables the repair device to maintain sufficient integrity while the soft tissue or bone heals. Surgical repair devices formed of materials which are absorbed too quickly may fail when compressive, tensile or flexural loads are placed on the devices before the tissue or bone has fully healed.
There is also a need for absorbable polymeric materials that can be permanently deformed at room or body temperature, especially in medical device applications that require the material to be reshapable. One such application is in absorbable maxillofacial bone fixation plates where complex fracture site surface contours are often encountered. For comparison, see the Masterpiece.TM. maxillofacial bone plate system (Storz Instrument Co., Mo. 63122, USA). Another application is in absorbable surgical clips and staples where improved toughness and ductility are desirable.
As the term is used herein, permanent deformation refers to a type of deformation wherein the material does not completely return to its original form once the deforming load is removed. Such a deformation results from the properties of ductility or plasticity. Ductility and plasticity have equivalent meanings as used in describing the invention and can be applied in elongational or flexural deformations. A ductile polymer "yields" when a sufficient stress or strain are applied. The yielding process can involve molecular reorganization (usually in softer polymers with glass transition temperatures below ambient temperature) or crazing (usually in more rigid polymers with glass transition temperatures higher than ambient temperature), discussed in more detail below.
The following U.S. patents are pertinent to the present inventions described in this application: 4,243,775, 4,279,249, 4,300,565, 4,539,981, 4,550,449, 4,744,365, 4,788,979, 4,839,130, 4,844,854. Also pertinent is the international patent application WO 89/05664 and corresponding U.S. Pat. Nos. 4,891,263, 4,916,193, 4,916,207 and 4,920,203. These patents and the application are incorporated herein by reference.
The modification of glassy polymeric materials for improved toughness is well known in the non-absorbable polymer art. Perhaps the most notable example of a toughened glassy plastic is high impact polystyrene (HIPS). The following review article describes the property improvements of HIPS: Soderquist, M. E. and Dion, R. P., "High Impact Polystyrene," in Encyclopedia of Polymer Science and Engineering, Vol. 16, pp. 88`97, John Wiley & Sons, New York, 1989. Many other nonabsorbable polymers have been modified for improved toughness or impact resistance. A general review of this field can be found in Yee, A. F., "Impact Resistant Materials," in Encyclopedia of Polymer Science and Engineering, Vol. 8, pp. 59-68, John Wiley & Sons, New York, 1989 and in Bucknail, C. B., Toughened Plastics, Applied Science Publishers, London, 1977 and in Comprehensive Polymer Science, Vol. 2, section 15.3, pp. 526-532, C. Booth & C. Price, eds., Pergamon Press, New York, 1989. Generally, toughness and impact resistance have been improved by incorporating a discontinuous rubbery phase in the parent polymer matrix. This has been done by physical blending or by preparation of block or graft copolymers. Similar concepts have been applied to thermosets such as epoxy resins (see Yee, A. F. and Pearson, R. A., "Toughening Mechanisms in Elastomer-Modified Epoxies Part 1 Mechanical Studies," J. Mat. Sci., Vol. 21, 1986, pp. 2462-2474 and Pearson, R. A. and Yee, A. F., "Toughening Mechanisms in Elastomer-Modified Epoxies Part 2 Microscopy Studies," J. Mat. Sci., Vol. 21, 1986, pp. 2475-2488. All of the above cited disclosures are incorporated herein by reference. Although increases of ductility in nonabsorbable rubber modified plastics have been reported, the primary purpose of the modification has been to impart impact resistance and toughness. To our knowledge this property modification method has not been put to use in medical devices, either absorbable or nonabsorbable.
The U.S. Pat. Nos. 4,243,775 and 4,300,565 cited above disclose absorbable polymeric compositions which were thought to form a two phase morphology. These patents do not mention any enhancement of deformability in bending due to the presence of a rubbery phase.
Some other patents (U.S. Pat. Nos. 4,744,365, 4,839,130 and 4,844,854) disclose two phase copolymers of lactide and glycolide. These patents do not mention any enhancement of deformability in bending other than reduced brittleness. Also, the copolymers disclosed in the '365, '130 and '854 patents do not contain a rubbery phase; rather, they contain two semicrystalline glassy phases. The utility of these two phase copolymers are described as a surgical clip or staple. The rubber toughened materials of this application may also be useful as a surgical clip or staple.
None of the prior art mentions the usefulness as a medical device of materials which can be permanently deformed at room temperature through crazing. The term "crazing" is used in its typical meaning of cavitation and/or microcrack formation, such as occurs during bending one portion of a suitable material relative to a second portion to form crazes at the bend site. Crazing will not occur if the material is too rubbery or too brittle. In some materials, sufficient crazing can take place to permit the material to deform ductily or plastically under an applied load.
U.S. Pat. No. 4,279,249 claims bioabsorbable boneplate devices manufactured from a copolymer of at least 90% units derived from lactic acid and reinforcing fibers made of polyglycolic acid or a copolymer thereof. Nowhere in this patent is it disclosed that the material can be permanently deformed by bending at room temperature, although improved resilience and shock resistance are disclosed. Also, this patent claims a "matrix" polymer of at least 90% lactic acid units.
Other bioabsorbable bone fixation devices have been fashioned from high molecular weight poly(1-lactide); see, e.g. U.S. Pat. Nos. 4,539,981 and 4,550,449. This material does not allow reshaping at room temperature and no mention of such a property is made in these patents.
Block copolymers containing trimethylene carbonate and lactide were exemplified in the U.S. Pat. No. 4,916,193. The materials exemplified were higher in rubber phase content compared to the compositions described in this application. In this patent no mention was made of ductile properties or the usefulness of such a property in medical devices.
Block copolymers containing trimethylene carbonate, caprolactone, and glycolide as well as block copolymers containing caprolactone and glycolide were exemplified for use as suture coatings in U.S. Pat. No. 4,788,979. In contrast to the materials of the present invention, these materials were also rich in soft phase forming units. No mention was made of ductile properties in this patent.